Laboratory tests - viral serology

Role

Viral serology in gastroenterology is used predominantly for hepatitis virus diagnosis/staging (see hepatitis B serology and viral hepatitis transmission for detail) and for excluding a viral cause in specific GI presentations (e.g. CMV colitis in the immunocompromised)
Central to the initial workup of any unexplained transaminitis or suspected viral hepatitis, and to staging known chronic viral hepatitis for treatment decisions

Hepatitis serology patterns (HBsAg, anti-HBc, anti-HBs, HBeAg for hepatitis B; anti-HCV antibody with confirmatory HCV RNA for hepatitis C) distinguish acute, chronic, resolved, and immune (vaccinated) states, each requiring a different management pathway
In immunocompromised patients with unexplained colitis or GI symptoms, CMV serology/PCR and tissue testing (biopsy with immunohistochemistry or PCR) can identify CMV colitis/reactivation as a distinct, treatable cause requiring antiviral therapy rather than escalation of immunosuppression alone

Any unexplained transaminitis should include hepatitis A, B, and C serology as part of first-line workup, alongside other common causes (alcohol, MASLD, autoimmune, drug-induced)
CMV colitis in an immunosuppressed patient (e.g. IBD on biologics/thiopurines) can mimic a flare of the underlying IBD - distinguishing the two (via tissue CMV testing on biopsy) is clinically important, since the correct treatment (antiviral therapy vs escalating immunosuppression) is opposite depending on the cause

Confirmed hepatitis B/C -> antiviral treatment per current guidelines and ongoing surveillance (e.g. HCC surveillance in appropriate patients)
Confirmed CMV colitis -> antiviral therapy (e.g. ganciclovir/valganciclovir), with immunosuppression reduction considered where feasible given the underlying immunocompromised state contributing to reactivation

Hepatitis B/C chronic infection and HCC surveillance, CMV colitis in IBD/immunosuppression, unexplained transaminitis workup

Mistaking CMV colitis for a simple IBD flare (and escalating immunosuppression rather than treating the CMV) is a recognised, harmful error in immunosuppressed IBD patients - active consideration and testing for CMV should be part of the workup for any apparent flare that is severe, atypical, or occurring on significant immunosuppression

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